Field of the Invention
This invention relates to impact sensors and more particularly to a passive impact sensor for high velocity impacts that transmits impact data off-projectile.
Description of the Related Art
Shock and impact sensors are devices that detect sudden movements, changes, or severe impacts at a predetermined level and indicate whether that level has been exceeded. Impact sensors are used in applications where it is desirable to know when an impact has occurred. In an ultra-high velocity or hyper-velocity impact, the relative velocities of the colliding objects can range from about 1,000 meters per second (m/s) to about 15,000 m/s with 5000 m/s being roughly the speed of sound in metal. The speed of sound through the metal construction materials of a projectile limits the propagation speed of the shock wave from an impact through the projectile. Under these conditions, the normal working assumptions of conventional technologies used in the art break down. An example of an application that uses this ultra-high velocity is an anti-projectile projectile. During the final flight stage of the projectile, high velocities are used to improve the accuracy and the efficacy of a successful engagement. In the case where it is desirable for a projectile to send a notification that it has had an impact, this event must be sensed, processed on-board, and transmitted after the impact has occurred, but before the projectile is destroyed by the impact. The projectile's impact sensor needs to be able to detect that an impact has occurred before the sensor is destroyed. Related to this, the sensor must be able to trigger a notification message be sent, and the message sent before the transmitter is destroyed.
Conventional techniques use electrical sensors to detect an impact. Conventional techniques require that the shock from the impact arrive at the sensor and the sensor actuates before the sensor is destroyed. If the electrical sensor is positioned at the anticipated area of impact, the sensor will be destroyed on impact, hence unable to send an impact notification message. Another option for an electrical sensor is to position the sensor in an area of the projectile that is not near the area of impact and detect an indication of the impact. This method is not sufficient at ultra-high velocities because the velocity of the destructive shock wave through the projectile structure exceeds the speed of the sound in the materials of which the projectile is constructed, so the sensor is destroyed before being able to detect the impact.
Another option is to use a conductive circuit positioned at the anticipated area of impact and an electrical sensor positioned in a second area of the projectile, away from the anticipated area of impact. An electrical signal, such as a voltage, is supplied through the conductive circuit. The sensor monitors the conductive circuit for a change in the signal being supplied to the circuit. When the projectile impacts, the conductive circuit is destroyed before the sensor is destroyed. When the sensor measures a change in the signal being monitored, the change can be analyzed, and if this change indicates that the circuit has been destroyed, the sensor can trigger an impact notification message. This technique is known in the art and is used to measure impacts at velocities about 1000 m/s, which is much lower than speed of sound in the materials of which the projectile is constructed. At these velocities, the impact results in damage to the conductive circuit, for example breaking of the conductivity of the circuit. The corresponding change, in this example, loss of signal in the circuit, is measured by the sensor, and an impact message can be sent before the sensor is destroyed.
This technique of using a conductive circuit is not sufficient to detect ultra-high velocity projectile impacts because of the type of destruction resulting from the impact. When there is an ultra-high velocity projectile impact, the construction material of the projectile transitions to an indefinite state. The unpredictable effects of an ultra-high velocity impact on electrical circuitry may be because of the possible formation of plasma, or other unpredictable physical phenomena, due to the velocity of the impact exceeding the speed of sound in the material. The operation of a conductive circuit under these conditions cannot be predicted reliably. The destruction of a conductive circuit at ultra-high velocities does not provide a reliable change in the signal. For example, the conductive circuit may short instead of breaking, or may have a non-repeatable response.
US Patent Pub 2010/0307353 entitled “Ultra-high velocity projectile impact sensor” discloses an apparatus for detecting the impact of an ultra-high velocity projectile including: a projectile; at least one optical fiber attached to at least a first area of the projectile; a light source coupled to the at least one optical fiber supplying light into the at least one optical fiber; and a monitor coupled to the at least one optical fiber configured to monitor a property of the light in the at least one optical fiber and positioned in a second area of the projectile. An optical fiber provides a predictable response under the conditions of an ultra-high velocity projectile impact. When the optical fiber is intact, it propagates light and when the fiber is damaged, the light decreases. In the case where the optical fiber is broken or destroyed or even under some conditions of shock and vibration, the light cannot propagate or propagation is decreased through the optical circuit. Referring to FIGS. 2A-2D, schematic examples of some options for optical fiber layout, the optical fiber may be deployed in a variety of configurations. FIGS. 2A-2C are examples of laying out the cable on the substrate in several optional configurations. FIG. 2D is an example of deploying more than one optical fiber cable on a substrate. It is also possible to use more than one substrate.
When a projectile strikes a target, the impact will be at a first area of the projectile. Depending on the design of the projectile, this first area will begin to crush, collapse, fragment, explode, or similar. Given the ultra-high velocity of the impact, high energies are involved and the materials at the first area of the projectile begin to transition to an indefinite and/or unpredictable state. The optical fiber in the first area is possibly deformed, then destroyed, resulting in an interruption to the light propagating through the optical fiber. The shockwave from the impact begins to travel through the projectile from the first area of impact toward the second area farther away from the impact. The velocity of light in fiber is significantly faster than even ultra-high velocity impacts of a projectile with a target. This difference in velocities allows the monitor to detect a change in the light at the second area before the shockwave reaches the second area and damages or destroys the monitor.
Because of the limitations of high velocity impact sensors, techniques for battle damage indication (BDI) are currently limited to off-board sensors that observe an impact of a projectile and a target, and use processing to characterize the impact. Sensor types may include radar systems, optical systems, radiation detectors, etc. These systems may be located on the ground, at sea, on aircraft, on spacecraft, or on satellites. Typical responses measured include the impact flash, the trajectories of the projective target both prior to and after impact, residual projectile and target motion, any physical breakup or fragmentation that occurs, and any radiation detected as a result of the impact. At best, these off-board impact measurement systems can infer information about the impact.